On the eve of the second Seattle visit of "Miss Saigon," one can't help but ponder the astonishing hardiness of such splashy mega-musicals.

Hatched in London in 1989, "Miss Saigon" was, in a sense, the last true blockbuster in a wave of whale-scale, semi-operatic musical entertainments to conquer the globe.

"Miss Saigon" is a production of Cameron Mackintosh, the savvy British impresario who also keeps "Les Miserables" and "Phantom of the Opera" in constant circulation.

And though it may not be as easy a sell as those other two Mackintosh perennials, or the ubiquitous "Cats," this Vietnam War spin on the "Madame Butterfly" story has overcome initial protests (against its use of non-Asian actors in Asian roles) to gross $900 million worldwide.

"We don't see any end in sight for it, or for `Phantom' and `Les Miz,' " reports Peter Lawrence, executive producer of "Miss Saigon" for the Mackintosh organization. " `Miss Saigon is currently booked through 2001. So is `Les Miz.' And `Phantom' will go on after we're all dead. It hasn't begun to wind down yet."

Between 1999 and 2000, Seattle will have had return visits by all three shows, plus "Cats." Drew Murphy, who runs the Broadway at

the Paramount series for Pace Theatrical, says demand remains high.

As of last week, for instance, "Miss Saigon" had pre-sold 40,000 tickets for a three-week run that's not part of the Paramount's regular theater subscription series.

"These shows aren't so much of an attraction to our subscribers - they've seen them before," says Murphy. "But I'll be first to admit that when they come around, you're almost guaranteed good business."

Though some of us would prefer a fresher lineup of music-theater attractions, it's undeniable that an old blockbuster sells more tickets than even some hot new Broadway hits.

For instance, Seattle runs of the award-winning "Ragtime," and a snazzy revival of "Chicago," didn't do nearly as well as a recent, umpteenth Paramount stand of "Les Miz" did.

Says Murphy, "These big shows become our `Nutcracker' or `Christmas Carol.' Sometimes a `Cats' can make up for any losses you may have acquired over a year. And this year we had a very rough summer."

He notes that a sure-thing smash also brings in "people who've never been to live theater before this. Hey, if `Cats' introduces more people to theater-going, we're happy."

Yet what's the intrinsic appeal of these well-hyped, "critic-proof" juggernauts?

Lawrence says it varies from show to show. "Miss Saigon," for example, "pulls in a younger crowd, because of the nature of its love story about a young Vietnamese woman and an American soldier. It's not just a musical about a chandelier or a helicopter. People love the spectacle, but that's not all there is to it."

Yet the sight of a very authentic-looking military helicopter landing onstage is certainly an indelible moment in Nicholas Hytner's elaborate staging of the piece, and in John Napier's dazzling production design.

After all, it's not star power that packs the balcony for a "Miss Saigon." Though the Mackintosh group is scrupulous about casting and quality control, the best-known name in the current national touring cast is Seattle native Greg Stone, who plays Chris, the romantic male lead.

And it's not necessarily the Alain Boublil-Claude-Michel Schonberg-Richard Maltby Jr. score of "Les Miz" that woos in the crowds either. The music has its devotees, but it's a fairly homogenized pop-opera blend with no stand-alone hits.

Whatever the magnet, it's likely that "Miss Saigon" (which reopened the renovated Paramount in its first run here, in 1995) will be back.

But how long can the old faithfuls keep circling the planet, and fueling the national theater economy? And when they do expire, what if any new cash cows will replace them? That's a question not even Cameron Mackintosh can answer yet.